QML Extention objects don't work with uncreatable types

I was trying to extend an object type that we get as a replica from QtRemoteObjects (using the .rep files). We already had the replica registered with QML as a foreign type for access to the enums in there. So, I added a QML_EXTENDED(MyTypeExtention) there, and... nothing. 

In order of what I would like to see to get this addressed, from least-desirable and easiest to most desirable but probably most involved:

1. There is no mention in the documentation about such a limitation. It seems that extension objects only work on types that you can instantiate in QML?
2. There is no warning at compilation or at runtime about there being something wrong. 
3. Ideally, it should just work. It would be a great way to decorate these objects, especially since we can't subclass from them either.